The invention relates to the field of multistage gearings used for power transmission by continuous, articulated or flexible traction-transmitting means such as a belt or a chain. More particularly, the invention is directed to a control mechanism for setting a transmission ratio between a traction means, e.g. a chain or a belt, and a wheel set that is rotatable about a wheel axle and includes two or more wheel blades. A selected one of the wheel blades is wrapped around by the traction means; at least one of the wheel blades is composed of a plurality of independently adjustable wheel rim sectors, wherein the control mechanism causes the adjustment of the wheel rim sectors relative to a fixed plane in which the traction means wraps around the wheel set, in a direction substantially transverse to said plane.
The invention is concerned with the idea to change the transmission ratio in power transmission devices comprising sectored, divided or interrupted drive and/or driven wheels—or blocks consisting of such wheel components. Such transmissions are, for instance, known in gearshifts of bicycles, wherein a transmission between the wheel and the traction means takes place twice, namely once from a crank wheel to a chain, and once from the chain to the drive wheel (which is usually the rear wheel of the bicycle).
Several solution principles are known for variable transmission ratios.
A first known approach consists in a derailleur gear. Such systems are sufficiently well-known (cf. e.g. U.S. Pat. No. 3,448,628). A derailleur guides the chain and moves it axially on the chain blades with different diameters, which are adjacently located on an axle. Thereby, the transmission ratio is changed relative to the chain movement, and consequently relative to a further chain blade aligned with the chain (which corresponds to the driven wheel or the power-output wheel).
In doing so, disadvantages frequently arise in that the chain is moved out of alignment, which will cause a higher friction in the chain, or a higher load on the chain, thus resulting in material fatigue. A further disadvantage consists in that the chain is partially present on two chain blades during the shifting procedure, which is why no power can be transmitted during that time. Shifting during load operation is thus not possible; rather has the driving power to be minimized during a shifting procedure in order to enable a shifting procedure to be performed without material damage. In order to realize large transmission ratios, i.e. enable chain blades having diameters largely varying in size to be arranged on an axle, it is necessary to arrange several chain blades increasing in terms of size in conical form so as to be usable as lifting aid by the chain, even though they will not actually be required for the transmission. In addition, vibrations may cause the chain to skip to adjacent chain blades, which will again cause problems during load operation.
Moreover, when providing two derailleur gears in a gear stage, i.e. both on the driven and on the power-output axles, in order to realize particularly versatile and/or large transmission ratios, it is to be made sure that the chain blades be selected such that the chain will not run too much out of alignment, since this would involve high material stress. Another disadvantage is that the derailleurs are usually located on exposed sites, as is frequently the case particularly in bicycles, and are therefore prone to damage during cross-country rides, e.g. by bushes. Due to the plurality of axially adjacently arranged chain blades, derailleur gears also require relatively much space, which will again prevent a gearshift having the full range of transmission ratios from being realized on the crankshaft, which is better protected, yet very limited in terms of space.
An alternative approach comprises gearshifts. There, a change in the transmission ratio is accomplished by a sprocket gear, for instance a planetary gear; the chain drive merely serves as a power-transmitting element. Such systems are primarily used in the rear-wheel hub of a bicycle. The drawback in that case is the high weight that such sprocket gears have, and the high constructive and economic expenditures.
Another alternative approach uses bevel gears and step or cone pulleys. In that case, substantially two stepped or conically shaped discs are provided, between which the traction medium runs. By changing the axial distance of the discs, the traction medium is forced onto different radii of contact thus changing the transmission ratio. This principle is only suitable for frictionally engaged, but not for positively engaged traction media such as chains or toothed belts.
By contrast, the invention is based on the idea of using multistep chain gears with sectored, divided and/or interrupted sprockets. Here, the drive or driven wheel under consideration is virtually divided into components that are successively brought into alignment with the chain. The traction means, by contrast, is moved in a fixed plane, a lateral movement of the traction means being not required or even prevented. In the following, the plane in which the traction means extends will be referred to as “plane of the traction means” or, briefly, “plane of alignment”.
FIG. 1 illustrates the already known principle underlying the present invention, of changing the transmission ratio by axially feeding sectors into the alignment of the traction medium. Shown is a wheel set 101 with three wheel blades 110, 120, 130 each corresponding to a drive or driven disc for a respective transmission ratio or “speed”. While the innermost wheel blade 110 is undivided and axially fixed, the central wheel blade 120 and the outer wheel blade 130 are each divided into sectors 102, 103 (usually corresponding to circular ring sectors), i.e. each into three sectors corresponding to an angle of almost 120° in the illustrated example. The angle is slightly smaller than 360°/n (here, n=3) in order to allow for a play between the individual sectors. The sectors 102, 103 are mounted so as to be relatively movable along the direction of the axis 105, yet rotationally fixed in respect to the rotation. Such mounting is realized via suitable means, e.g. a pivot guide 104 or the like.
From FIG. 1, the start of a shifting procedure from the speed of the smallest wheel 110 to that of the central wheel 120 is apparent. The chain 106 still wraps around the wheel blade 110. One or more guide rollers 107 serve to enlarge the angular region that is not wrapped around by the chain 106. A first sector 102a of the central wheel blade 120, i.e. the one which is currently located in the unwrapped angular region, is coupled into the alignment of the chain 106. During the rotary movement of the wheel set, sector 102a will receive the chain. After this, the remaining sectors of the wheel blade 120 can be similarly coupled into alignment with the chain as soon as they have each reached the respective angular region that is not wrapped around; after this has been achieved for all sectors 102, the change to the speed corresponding to the central wheel 120 is completed.
This approach, which is based on the use of sectored, divided and/or interrupted wheel rims, overcomes the drawbacks of the previously mentioned approaches. Shifting during load operation is possible, since the chain is in engagement with the chain blades of both transmission ratios (and only with these) even during the shifting procedure. The space demand of such gearshifts can be reduced.
Some solutions are based on the principle of temporarily displacing the divided, sectored or interrupted sprockets only during the shifting operation proper, in order to “take” the chain onto the next-larger or next-smaller diameter. Such solutions are, for instance, described in U.S. Pat. Nos. 4,127,038 or 4,580,997. There, the chain is aligned with the next chain blade by pivoting in, or axially moving, the adjacent larger or smaller chain blade sector. The alignment of the chain will thus change at a change of the transmission ratio.
CH 617 992 A5 shows a principle by which the chain blade sectors are gradually fed into the alignment of the chain. The chain blade sectors are individually mounted on a co-rotating device by the aid of pins. Thereby, a smaller construction has become possible. This basically enables the construction of a derailleur gear providing the whole spectrum of transmission ratios just on a single axle.
All of the presently known principles have in common that the movement of the chain blade sectors is achieved by a contact of the rotating part, e.g. via pins, with a fixed part, i.e. a part immovable relative to the rotating unit on which the sectors are located, e.g. a radially movable carriage. This involves the disadvantage of a constant contact, and hence friction, persisting during the operation of the system. In addition to an undesired load and wear, this will also lead to the generation of noise. This renders such principles uncomfortable in practice and does not constitute a viable solution.
That known solution involves the problem of how the feeding of the chain blade sectors into the alignment of the chain by the actuation of individual pins takes place. Since every sprocket sector has to be mounted on two or more pins, the latter will tend to jamming if only one pin is actuated, and render impossible any movement of the toothed sectors. In order to avoid this, CH 617 992 A5 also shows the mounting of quarter-circle sectors on three pins each, and a mechanism that is to enable the pivoting-in of the pins (as opposed to a parallel guidance of the sectors). The guidance and meshing of three pins per sector, however, involves the disadvantage that an excursion of a sector is not possible without inacceptable expenditures. Another problem relates to how the individual sectors are to maintain their positions in or out of engagement after having left the stationary guide carriage. In this respect, none of the known solutions offers an approach that is effective in practice.
A variant of the last-discussed approach is also described in CH 617 992 A5, namely that the sprocket sectors of varying sizes are mounted on a common, radially arranged pin and pivoted into alignment with the chain. This solution does not involve the problem of jamming of the guide pins. However, what is disadvantageous is the relatively high space demand, since the individual sectors have to be pivoted out at relatively large angles in order to completely leave the region of the running chain. In addition, this solution involves the drawback of the force for a gear change having to be introduced by contacts between stationary and rotating parts. This will again cause the already mentioned friction and noise generation.